“Scientific truth and natural phenomena are as good subjects for art as are man and his emotions,” wrote Berenice Abbott, an acclaimed American photographer who turned her documentary skills to the illustration of mathematical principles governing the physical world. Her photographs of soap bubbles were shot in 1940, when she was the picture editor of Science Illustrated. The bubbles shimmer with intensity as they huddle, minimizing the total area of their exposure to the nonbubble world.
The bubble is the Clark Kent of the natural world: mild-mannered and a bit of a wimp--until it strips for action. A preview of the awesome power of the seemingly innocent orb came in 1917 when English physicist Lord Rayleigh revealed how bubbles in ocean water cause the mysterious erosion of the metal in ship propellers. Today the bubble is being harnessed in a dazzling array of industrial and medical uses. Someday it may even help save your life.
Oil refiners employ bubbles in the removal of sulfur from crude. Bubbles are used in ink-jet printers to force inks through microscopic nozzles. Plastic surgeons use ultrasound to create bubbles that liquefy fat they wish to remove through liposuction. In the future, bubbles may sterilize surgical instruments and break up kidney stones. And some physicists speculate that the heat of bubbles targeted by sound waves is theoretically sufficient to trigger fusion, a Holy Grail of renewable energy.
None of this detracts from the bubble’s charm—so natural that its essential message is easily missed. A bubble wafted by a summer breeze, its spherical shape enclosing a maximum volume of gas for a given area, is a perfect demonstration of the law of conservation of energy. In two dimensions, bubble walls trace the shortest possible distance between a series of points. Bubbles minimize their own area by settling into a mosaic of hexagons—a pattern accepted for the past century as using the least energy, but definitively proven only this spring. Mathematician Thomas Hales of the University of Michigan in Ann Arbor cracked the hexagonal problem but remains mystified by another bubble puzzle: Which bubble shapes most efficiently fill a room? “I’m working on it next, but it won’t be found in my lifetime,” Hales says. Transparent and ephemeral, bubbles still guard their secrets.
|CONTAINED COLLAPSE |
Cavitation—bubble implosion—can make pieces of steel look as if they’ve been pummeled by artillery. An increase in internal pressure or a drop in the pressure of the surrounding fluid bloats a bubble to a hundred times its original size and creates an almost perfect vacuum inside. When normal pressure returns, the bubble collapses violently; if it’s on a surface, this launches a minuscule dagger of water through its center (like the bubble, center left, magnified 45 times). A cavitating bubble on a ship propeller, for instance, sends its water jet smashing into the metal at up to 560 mph. Although the bubbles involved are at most TK microns high and the entire cycle lasts only 50-millionths of a second, the continuous pounding can ruin a new propeller in a matter of days. But the fury of cavitation may be put to positive use someday soon. Ultrasound targeted at specific sites in the body, such as kidney stones or tumors, can form tiny bubbles that become micron-size jackhammers destroying a growth without drugs or surgery.
Shock waves, like the yellow arc created by a .22-caliber bullet passing through a helium-filled soap bubble, can produce mysterious effects. In a process called sonoluminescence, sound waves cause an air bubble in water to pulsate 20,000 times a second. Each collapse can create a miniature sonic boom, which emits a tiny white glow believed to be much hotter than the surface of the sun. Such temperatures could theoretically be harnessed to produce fusion.
Two soap films separated by a thin layer of water create a psychedelic bubble wall. Because some rays of light bounce off the inner layer of film and some off the outer, waves in the color spectrum fall out of sync and cancel one another. Thick walls cancel red, leaving only shimmery blue. Magenta swirls occur when walls thin and green vanishes. When black appears, the wall is so thin it’s about to pop.
|ASCENDING ORDER |
A queue of rising air bubbles in water soon becomes disorderly. A series of time-lapse photographs, just 35-thousandths of a second from start to finish, shows the slipstream of the flatter leading bubble accelerating the progress of its follower. A few instants later, the two merge.
When studying vortices—the turbanlike swirls that shear away at the edges of a moving fluid—physicists can rely on bubbles to help them visualize the eddies. An electrically charged wire (bottom center, three times actual size) produces tiny hydrogen bubbles that act like bread crumbs marking the spiraling pathways of a jet of water. Such bubble-generated models are crucial in the design of airplane wings. Overly massive vortices impede ascension, causing a plane to plummet.
|UPWARD MOBILITY |
Air bubbles follow unpredictable twisting paths as they rise in moving water. Using long exposures, Australian physicist Richard Manasseh hopes one day to foretell the patterns bubbles follow, and the amount of time they need to reach the surface. This could help climatologists anticipate the pace of global warming. Whitecaps, waves, and rain drive air bubbles containing greenhouse gases nearly 30 feet into the ocean. Then as the bubbles ascend, the gases dissolve into the water. How much greenhouse gas is removed from the atmosphere depends on how long the bubbles remain submerged.
Hidden beneath the gaudy exterior of a bubble hive is an entire Principia of cluster geometry, lessons extending far from the bathtub or the kitchen sink. For example, a city planner wanting to determine the roadway connections between three towns would do well to check out the merger of three bubbles, which invariably meet one another at angles of 120 degrees, demonstrating the shortest possible distance between three points. Bubbles may be able to entertain with wonder and wow, but they are also loaded with wisdom.
|MASKED INTRUDER |
Bubbles would prefer to be droplets—much less area to keep intact and therefore much more energy efficient. One pinhole in its surface is all the excuse a bubble needs to collapse. But a pencil dipped in soapy water (you can try this at home) will fool a bubble into thinking that the intruder belongs. Makes a good handle for bubble transport.
Dolphin Bubble Ring Gallery
Fluid Dynamics Research
Simulations of Bubbles Rising
The Exploratorium's Bubble Page